1. Field of the Invention
The present invention relates to a MOSFET switch module and an LED driving circuit that utilizes the MOSFET switch module; in particular, to a MOSFET switch module with voltage clamping function and an LED driving circuit that utilizes the MOSFET switch module.
2. Description of Related Art
Referring to FIG. 1, which shows a prior art LED driving device. The LED driving device comprises a dimmer control unit 10, an error amplifier 15, an n-channel metal-oxide-semiconductor field-effect transistor (NMOS) 20, a power supply 25, an LED module 30, and a current detecting resistor R. The power supply 25 couples to one end of the LED module 30 so as to provide a driving voltage for driving the LED module 30 to emit light. The NMOS 20 is couple to the other end of the LED module 30, and controls the amount of current flowing through the LED module 30 according to a switch control signal. The current detecting resistor R is coupled to the NMOS 20, so as to detect the amount of current flowing through the LED module 30, and generate a current detection signal. The error amplifier 15 receives the current detection signal from the resistor R and a control signal from the dimmer control unit 10, and according to these two signals outputs the switch control signal to the NMOS 20, so as to control the amount of current flowing through the NMOS 20. The dimmer control unit 10 may receive a dimmer signal, and when the dimmer signal indicates ON status, generates a reference signal as the control signal. The error amplifier 15 makes a voltage level of the current detection signal from the resistor R equal to that of the reference signal of the dimmer control unit 10, and the NMOS 20 is thereby under ON status. On the other hand, when the dimmer signal indicates OFF status, the dimmer control unit 10 generates a low level signal as the control signal, so that the NMOS 20 is under OFF status. According to the dimmer process mentioned above, the dimming effect is thereby achieved.
When NMOS 20 has been cut off (OFF status), current no longer flows through the LED module 30, and at this time, the voltage of the drain of the NMOS 20 has been pulled to the level roughly equal to the driving voltage. By using 4 volt driving voltage of single LED as an example, when the LED module 30 has 20 LEDs connected in series, the overall driving voltage is 80 volt. Therefore, the NMOS 20 must be MOSFET with high withstand voltage characteristic. Due to this high withstand voltage requirement, the die size of the MOSFET needs to be larger and so the cost is greater, and additionally the gate-source capacitance (Cgs) also be increased. The increase of the gate-source capacitance (Cgs) means that the error amplifier 15 must also require high driving capability so as to drive NMOS 20. Furthermore, the power consumption of the MOSFET is in direct proportion to fCV2, wherein f is the switching frequency, C is the capacitance value of the gate-source (i.e. Cgs), and V is the voltage change from switching. Therefore the higher requirement of withstand voltage for a MOSFET, the higher power consumption, and also a heat dissipation circuit may have to be added so as to prevent the MOSFET from overheating and damaged.